Photoaddition of Dimethoxycoumarin to Thymidine: A mixture of DMC and thymidine (mole ratio 1:50) were irradiated with 350 nm light in frozen aqueous solution and in solid film state. The absorbance at 324 nm decreased more rapidly than the case of irradiation of DMC alone. TLC analysis revealed 2 cross addition products, one as a major product. A thin solid film gave the best results giving the largest quantity of the major product and benzene-acetone (2:3 v/v) solvent was the best eluent. The isolation and structure determination of photoaddition products of DMC to thymidine are in progress. The major photoadduct is very sensitive to 254 nm UV light giving DMC and thymidine and the adduct may very likely be a C4-cycloaddition product. The isolation of the product in large enough quantities for spectroscopic (UV, IR, NMR, Mass) studies and elemental analysis is in progress by TLC and column chromatography. The structure of the cycloadducts will be a good model for furocoumarin-DNA photoaddition product and will be a great help in understanding the mechanism of photosensitization activity of furocoumarins.